1. Field of the Invention
This invention relates to a process for sulfur removal from gas streams which comprise reductive sulfur compounds and carbon oxides by photosynthetic bioconversion forming elemental sulfur and organically fixed carbon compounds. The process provides greater than 90 to 99 percent sulfur removal with simultaneous organic hydrocarbon production by contacting such gas streams with photosynthetic sulfur bacteria under anaerobic conditions with electromagnetic irradiation. The process is particularly well suited for removal of hydrogen sulfide from acid gas containing streams, such as produced in fossil fuel hydrogasification or hydroliquefaction processes or in hydrogen sulfide removal from natural gas.
2. Description of the Prior Art
Many chemical processes, such as fossil fuel conversion processes, produce effluents that contain sulfur compounds, usually predominantly hydrogen sulfide. Natural or synthetic pipeline gases usually contain hydrogen sulfide which must be removed prior to transmission due to its poisonous nature. The hydrogen sulfide must be removed, in the case of fuels, in order to meet sulfur oxide emission requirements when the fuel is burned.
An important process for sulfur recovery has been the Claus process and various modifications thereof as described widely in literature, such as Wall, J. et al, "NG/LNG/SNG Handbook", Hydrocarbon Processing, pgs. 90-102 and 107-116, April, 1973; Dearson, J. J. "Developments in Claus Catalysis", Hydrocarbon Processing, pgs. 81-85, February, 1973; Kirk, A. T. and Othmar, B. A. Encyclopedia of Chemical Technology, Vol. 19, John Wiley & Sons, New York, 386, 1969; and Atwood, R. G., D. C. Swaim Jr., and C. M. Yon, "New integrated UCAP Process treats low - H.sub.2 S streams, trims emission", Oil and Gas Journal, 77, pgs. 111-114, 1979. Claus sulfur removing processes have been able to attain 90 to 95 percent sulfur recovery under ideal operating conditions with 3 and 4 stage units claiming up to 97 percent recovery. To achieve higher degrees of recovery and to meet air pollution standards has required installation of add-on tail-gas treating units. The Claus process has been applied to acid gas streams containing varying amounts of hydrogen sulfide, but is relatively inefficient in its operation at lower concentrations of hydrogen sulfide as evidenced by the requirement of the tail-gas cleaning processes which have been described in Slack, A. W. and G. A. Hollinden, "Sulfur Dioxide Removal from Waste Gases", Noyes Data Corporation, Park Ridge, N.J., pg. 165, 1975. The Claus process has been most effective on feed streams containing at least 15 mole percent hydrogen sulfide as more fully reported in Fleming, D.K., "Acid-gas removal systems in coal gasification" Symposium on Ammonia from Coal, National Fertilizer Development Center, Tennessee Valley Authority, Muscle Schoals, Alabama, May 8-10, 1979. Another process is the Stretford process developed for the removal of hydrogen sulfide from coal gas and its conversion to sulfur as described in Ellwood, P. "Metavanadates Scrub Manufactured Gas", Chemical Engineering, 71, pgs 128-130, July 20, 1964; and Visan, Srini, "Holmes-Stretford Process Offers Economic H.sub.2 S Removal", The Oil and Gas Journal, 76, pgs. 78-80, Jan. 2, 1978. Another process for high removal of hydrogen sulfide from gas streams, particularly those with low initial hydrogen sulfide concentration and/or high carbon dioxide/hydrogen sulfide ratios, is the Takahax sulfur recovery process as described in Wall, J. et al, supra. Another sulfur recovery process for continuous removal of hydrogen sulfide from natural gas or synthesis gas is the Giammarco Vetrocoke process described in Wall, J. et.al, supra. A more complete review of these processed is found in Cork, D. J., "Bioconversion of coal acid gas to biomass and chemicals", Midwest Energy Conference on Liquid Fuels from Coal and Biomass, Midwest Universities Energy Consortium, Ohio State University, Columbus, Ohio, October 5-6, 1981.
Removal of undesired sulfur compounds from gas and liquid streams by various microorganisms has been previously recognized. U.S. Pat. No. 1,701,825 teaches oxidation of hydrogen sulfide and its removal by Thiobacillus which oxidizes the hydrogen sulfide to sulfuric acid. The reduction of sulfate to sulfide ions by Desulfovibrio and breaking down of organic carbon containing materials has been suggested in dilute aqueou streams as taught by U.S. Pat. No. 4,200,523; oil retort water as taught by U.S. Pat. No. 4,124,501; waste water and oil formations as taught by U.S. Pat. No. 3,105,014; and sulfur containing ores as taught by U.S. Pat. No. 3,020,205.
Sulfate removal by sulfate reduction using Desulfovibrio biological cultures established mutualistically with either Chromatium or Chlorobium for production of elemental sulfur has been suggested. Lactic acid and raw sewage are taught as being suitable carbon sources for the sulfate reduction and cell growth. In this work a pure biological culture of Desulfovibrio converted Sulfate and lactic acid to hydrogen sulfide and carbon dioxide, respectively. The Chlorobium converted 88 percent of the H.sub.2 S to elemental sulfur. This and similar work is more fully described in Cork, D. J., M. A. Cusanovich, "Sulfate Decomposition: A Microbiological Process", pgs. 207-221, Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena, L. E. Murr, A. E. Torma, and J. E. Bierley, eds., Academic Press, Inc., New York, 1978, and Cork, D. J., M. A. Cusanovich, "Continuous Disposal of Sulfate by a Bacterial Mutualism", pgs. 37-48, Developments in Industrial Microbiology, Society for Industrial Microbiology, 1979. Under static batch conditions Chlorobium have been found to metabolize a maximum of about 4 to about 8 milli-Moles hydrogen sulfide per gram bacteria in as long as a two week period. Kinetics of Sulfate Conversion to Sulfur by a Bacterial Mutualism: A Hydrometallurgical Process, Cork, D. J., PhD Dissertation, University of Arizona, Tucson, 1978.
It has not, however, been previously recognized that the Chlorobium microorganisms can advantageously live in a hydrogen sulfide-carbon dixide gas continuously purged environment such as derived from an acid-gas effluent stream, nor has it been recognized that in a hydrogen sulfide-carbon dioxide gas environment there can be very rapid growth of Chlorobium with high rate simultaneous production of elemental sulfur and organic fixed carbon compounds from gas streams comprising about 0.1 mole percent to as high as about 40 to 65 mole percent hydrogen sulfide and in the presence of large excesses of C0.sub.2 to produce in excess of 95 percent hydrogen sulfide removal, and usually in excess of 99 percent removal. In accordance with the present invention more than about 45 milli-Mole H.sub.2 S and up to the range of 80 milli-Moles H.sub.2 S are metabolized per gram of photosynthetic sulfur bacteria in relatively short detention times.
Various aspects of the Cork process using Chlorobium microorganisms for removal of hydrogen sulfide from fossil fuel acid gas has been described in Cork, D. J., "Acid Waste Gas Bioconversion--An Alternative to the Claus Process", paper at 23rd Annual meeting of Society for Industrial Microbiology, Richmond, Va., Aug. 9-14, 1981 and published in Developments in Industrial Microbiology, Society for Industrial Microbiology, 23, pgs. 379-387, 1981; Cork, D. J., "Bioconversion of Coal Acid Gas to Biomass and Chemicals" paper at Midwest Energy Conference on Liquid Fuels from Coal and Biomass, Midwest Universities Energy Consortium, Columbus, Ohio, Oct. 5-6, 1981; and Cork, D. J. and Sauchen, M. A., "Bioprocess for Fossil Fuel Acid Gas Bioconversion--An Alternative to the Stretford Process" paper at 4th Symposium on Biotechnology and Energy production and Conservation, Gatlinberg, Tenn., May 11-14, 1982.
U.S. Pat. No. 4,135,976 teaches Chlorobium to be suitable to concentrate silver from waste photographic fixing solutions containing silver thiosulfate complex salt by application of the photosynthetic sulfur bacteria under anaerobic conditions with irradiation of light. To attain the growth conditions for the bacteria, municipal sewage or the like must be added to the waste fix solution. The process provides for the recovery of silver together with the purification or decontamination of the photographic processing effluent.